The present invention pertains to intrusion detection systems and in particular to coaxial strain sensitive cables responsive to energy applied thereto to provide an output signal indicative of the presence of an intruder when used in such systems.
Intrusion detection is becoming more important, both in military and civilian applications. Military bases and other installations, e.g. factories, power stations, stores and even private homes, are being protected by sophisticated electronic surveillance systems which are capable of monitoring the periphery of such a facility to provide a manifestation or warning indicative of the presence of an intruder. One type of such an intrusion detection system incorporates a sensing element in the form of a coaxial cable concealed in the ground and disposed about the outer edge or perimeter of the area to be protected. The sensing element is sensitive to the presence of the intruder of whatever type and is responsive to the movement of a person or object approaching the periphery, to produce a manifestation indicating such an intrusion.
In the prior art, intrusion detection sensors have been developed which recognize magnetic field surface disturbances to intercept and detect non-metallic objects moving across the periphery of the area to be guarded. An example of the patents disclosing such sensors in the form of a cable is U.S. Pat. No. 4,001,745 of Goodman. Such a magnetic field surface disturbance sensor typically includes a central core extending along an axis having a plurality of wires formed as coils disposed about the core with a jacket disposed to protect the entire aforementioned assembly. It is disclosed that the coils are wound with a predetermined tightness about the core dependent upon the expected seismic sensitivity to prevent extraneous activity, such as thunder or explosion, from producing a magnetostrictive signal. Similar sesnors are also disclosed in U.S. Pat. No. 4,166,264 of Starr and U.S. Pat. No. 3,747,036 of Erdmann.
Further, U.S. Pat. No. 4,206,451 of Kurschner discloses a system adapted to receive the output of a sensor as disclosed in U.S. Pat. No. 3,747,036 to filter and supply the output to a logic circuitry, which detects the amplitude of the input signal and its zero crossing history to determine whether the magnetically detected signal is that of an intruder. A further type of cable sensor is disclosed in U.S. Pat. No. 3,610,808 of Horwinski. This cable sensor includes a coaxially configured wire braid made of close woven wires, each such wire being insulated from the other and connected in a grid to form a continuous trigger circuit to be connected to a detection device providing an indication of any alteration of the trigger circuit characteristics. Thus, if excessive stress breaks one of the wires of low ductility of the trigger circuit, the detection device provides an indication of an abnormal condition.
In addition to those sensors which sense a change of the magnetic field or the establishment of an open or short circuit condition within a cable, there are strain sensitive coaxial cables comprising a center conductor, and a first dielectric layer and a second conductor in the form of a braided wire or a solid cylindrically shaped conductor both disposed coaxially about the center conductor. This strain sensitive cable is provided with a protective shield and is responsive to the application of stress as by an intruder to provide a signal upon the central conductor which may be detected to provide an alarm indicating the intruder's presence. Although the precise nature of the operation of such cables is not fully understood, it is theorized that the application of a pressure or force to such a coaxial cable results in relative movement between the second coaxial conductor to establish a charge upon the first dielectric layer. The charge is capacitively coupled to the center conductor to produce a signal therein that may be detected by a detection circuit to provide an alarm indicating the presence of an intruder. Problems have developed with such coaxial cables, whereby spurious output signals from the center conductor may be produced even in the absence of an intruder. It is believed that due to the structure of the solid or braided second conductor, that the second conductor does not move freely with respect to the first dielectric layer with the result that as the cable or associated structure relaxes with age, i.e. strain relief, relative movement occurs between the second conductor and the first dielectric layer to produce an undesirable spurious output signal.
U.S. Pat. No. 3,963,854 of Fowler and U.S. Pat. No. 4,131,758 of Felkel each relate to coaxial, shielded cables for transmitting data or power as opposed to sensing strain to provide an output signal indicative thereof. The significance of each of these patents is that it relates to cables having a center conductor about which there are disposed helically wound conductors. However, neither of these references disclose a cable that is adapted to sense the imposition of strain thereto and to permit relatively free movement between its helically wound conductors and its dielectric layer covering the center conductor.